1. Field
Example embodiments relate to an optical device including a three-coupled quantum well structure, and/or to an optical device including a three-coupled quantum well structure, which may improve the light absorption intensity in a multiple quantum well structure without increasing a driving voltage.
2. Description of the Related Art
3D cameras typically have not only a general image capturing function, but also a function of measuring a distance from a plurality of points on a surface of an object. A variety of algorithms for measuring the distance between an object and a 3D camera have recently been suggested. A typical algorithm is a time-of-flight (TOF) algorithm. According to the TOF algorithm, illumination light is emitted onto an object, and a flight time until the illumination light reflected from the object is received by a light-receiving unit is measured. The flight time of illumination light may be obtained by measuring a phase delay of the illumination light. A high-speed optical modulator is used to accurately measure the phase delay.
An optical modulator having superior electro-optical response characteristics is typically used to obtain a 3D image with high distance accuracy. Recently, a GaAs-based semiconductor optical modulator is used. The GaAs-based semiconductor optical modulator has a P-I-N diode structure in which a multiple quantum well (MQW) structure is disposed between a P-electrode and an N-electrode. In the structure, when a reverse bias voltage is applied between the P-N electrodes, the MQW structure forms excitons in a particular wavelength band and absorbs light. An absorption spectrum of the MQW structure characteristically moves toward a long wavelength as a reverse bias voltage increases. Accordingly, a degree of absorption at a particular wavelength may vary according to a change in the reverse bias voltage. Thus, according to the above principle, the intensity of incident light having a particular wavelength may be modulated by adjusting the reverse bias voltage that is applied to an optical modulator.
In the optical modulator, a distance accuracy increases as a contrast ratio, for example, a demodulation contrast, indicating a difference in the degree of absorption between when a voltage is applied and when the voltage is not applied, increases. Driving at a low voltage is advantageous to reduce or prevent performance deterioration due to heat. In general, an increase in the contrast ratio may be achieved by increasing the light absorption intensity and transition energy in the MQW structure. The light absorption intensity is inversely proportional to the thickness of a quantum well layer and is proportional to a value obtained by normalizing a square of a degree of superimposition between a hole's wave function and an electron's wave function in the quantum well layer by a sum of the area of each wave function. Also, transition energy that indicates a degree of an absorption spectrum moving toward a long wavelength is proportional to the fourth power of the thickness of a quantum well layer and to the square of an applied voltage.
However, when the thickness of a quantum well layer is reduced to increase the light absorption intensity, the transition energy decreases and an applied voltage increases in order to compensate for a decrease in the transition energy. Reversely, when the thickness of a quantum well layer is increased to increase the transition energy, the degree of superimposition between a hole's wave function and an electron's wave function decreases, and the generation of excitons by electron-hole pairs is reduced so that absorption intensity decreases. Thus, since a high absorption intensity and a low driving voltage are in a trade-off relationship, it is difficult to simultaneously achieve improvement of the absorption intensity and reduction of the driving voltage.
In a TOF-type 3D camera, light having a wavelength of about 850 nm in an infrared range is generally used as illumination light. Since a GaAs substrate is not transparent with respect to the 850 nm wavelength light, a complicated process of removing the GaAs substrate is added to the process of manufacturing an optical modulator. Recently, to omit the complicated additional process, there have been efforts to use light having a wavelength of about 870 nm or more, for example, about 940 nm, which transmits through the GaAs substrate, as illumination light. However, since a lattice constant of the material of a quantum well layer and a barrier suitable for an optical modulator having a 940 nm resonance wavelength does not match the resonance wavelength of the GaAs substrate, stress and strain may be generated. Unless the stress and strain are compensated, quantum wells of a large number of layers are not stacked and thus it is difficult to increase the absorption intensity of an optical modulator.